1. Field of the Invention
The present invention relates to a thermally sensitive recording medium that has an excellent light resistance, and excels in appearance of unrecorded portion and color developing ability, further, prevents adherence of depositions and occurrence of sticking.
2. Description of the Prior Art
In general, a thermally sensitive recording medium is obtained by following procedure. Namely, a colorless or pale colored basic leuco dye and an organic color developer such as phenolic compound are respectively ground to fine particles and mixed together, a binder, a filler, a sensitizer, a lubricant and other additives are added, and a coating is obtained. The obtained coating is coated on a substrate such as paper, synthetic paper, film or plastic, thus a thermally sensitive recording medium is obtained. The thermally sensitive recording medium develops color by an instantaneous chemical reaction by heating with a thermal pen, a thermal head, a hot stamp or a laser ray, and a recorded image can be obtained. These thermally sensitive recording media are widely used in a field of a recorder of measuring equipment, a terminal printer of computer, a facsimile, a bar cord labeling machine and an automatic ticket vending machine.
Recently, as the applications of a thermally sensitive recording medium, uses for various kinds of caption label, indication medium such as poster and ticket are becoming more popular. However, since the stabilities to light and to heat of these kinds of thermally sensitive recording media are low, the occurrences of following problems are pointed out. That is, when said thermally sensitive recording medium is exposed to the room light or to the sun light, or left in high humid condition for long term, the background color of said medium changes to yellowish color and the appearance is spoiled, or the stability of recorded image is deteriorated, therefore, the commodity image of the thermally sensitive recording medium is remarkably damaged.
For the purpose to improve the light resistance of a thermally sensitive recording medium, a method to contain an inorganic filler that has ultraviolet ray screening effect in a thermally sensitive recording layer or a protecting layer which covers said thermally sensitive recording layer is described in Japanese Patent Laid Open Publication 62-18626 or in Japanese Patent Laid Open Publication 6-64324. However, in a case of inorganic filler that has ultraviolet ray screening effect such as zinc oxide, titanium dioxide or cerium oxide, although the ultraviolet ray screening effect of these compounds is high, the transmittance of visible radiation is low. Therefore, the color developing part is concealed and the printed image density becomes low, and if the containing amount of these compounds is reduced to prevent the occurrence of mentioned problems, the screening effect for ultraviolet rays is deteriorated and becomes not practical. Further, in a case of cerium oxide, since cerium oxide itself has light yellowish color, it is not suited as the material for a thermally sensitive recording medium.
Meanwhile, for the purpose to improve the light resistance, the method to contain fine ground particles of an ultraviolet ray absorbent in a thermally sensitive recording layer or a protecting layer which covers said thermally sensitive recording layer is disclosed (Japanese Patent Laid Open Publication 50-104650, 55-55891, 55-93492 and 58-87093). However, for the purpose to obtain a sufficient light resistance by containing fine ground particle of an ultraviolet absorbent in a thermally sensitive recording layer, it is necessary to add large amount of ultraviolet ray absorbent, because the absorbing effect and screening effect for ultraviolet rays are poor. Therefore, when these kinds of thermally sensitive recording media are kept in high temperature condition, the new problems such as background fogging or deterioration of the recorded image are caused, and the properties necessary for the thermally sensitive recording medium are damaged.
Further, when fine ground particles of ultraviolet ray absorbent are contained in a protecting layer, and if the ultraviolet ray absorbents are conventional type, these agents have low melting points and can be easily molten by the heat of a thermal head, and cause the problem of fine adherence of depositions or occurrence of sticking that stains the thermal head by long time printing. Furthermore, the ultraviolet ray absorbent is extracted from the protecting layer by the effect of plasticizer or oil, and consequently, the preserving ability of the recorded Image is deteriorated. And, in a case when the aqueous ultraviolet ray absorbent is used, as described in Japanese Patent Laid Open Publication 7-17131, since ions such as sodium salt are formed at the dissolving process of aqueous ultraviolet ray absorbent, the defect that the thermal head is damaged electric chemically is pointed out.
Further, the method to obtain a thermally sensitive recording medium that has good preserving stability and light resistance by containing microcapsules of ultraviolet ray absorbent in a protective layer is disclosed in Japanese Patent Laid Open Publication 5-155134. However, in a case of this method, the preparation of microcapsule in which fine particles of ultraviolet ray absorbent are immobilized is necessary, and to improve the absorbing efficiency of ultraviolet rays, it is necessary to make the size of microcapsule smaller. However, the size of microcapsule is restricted by the preparation method and the size of microcapsule has the smallest limit. Therefore, to obtain a sufficient light resistance, the necessary amount of the microcapsules that contain ultraviolet ray absorbent to add is increased, and the cost for the manufacturing of the thermally sensitive recording medium rises. Further, since the microcapsule in which ultraviolet ray absorbent is immobilized must be stabilized to heat and to pressure, the wall thickness of microcapsule must be thicker, and it is one of the ground of cost rising.
Recently, a thermally sensitive recording medium, which uses an ultraviolet rays absorbent prepared by polymerization of benzotriazole molecule or benzophenone molecule which has an ultraviolet ray absorbing ability, is proposed in Japanese Patent Laid Open Publication 7-314894, Japanese Patent Laid Open Publication 9-221487, Japanese Patent Laid Open Publication 9-268183, Japanese Patent Laid Open Publication 9-314496, Japanese Patent Laid Open Publication 10-71770 and Japanese Patent Laid Open Publication 10-36371. Still more, the thermally sensitive recording medium containing an ultraviolet ray absorbent of an emulsified copolymer composed of an ultraviolet ray absorbing monomer, a vinyl compound monomer which can copolymerise with said monomer and a bridgeable monomer in a thermally sensitive recording layer is described in Japanese Patent Laid Open Publication 6-73368. However, when the thermally sensitive recording media in which the compounds disclosed in these publications for patent are used, various problems mentioned below are caused, and the ultraviolet ray absorbent which has well balanced quality has not yet been obtained. Namely, problems of conventional ultraviolet ray absorbents for example; the appearance of unrecorded portion of thermally sensitive recording media are deteriorated because the color of materials itself is not so good; when used in a thermally sensitive layer, the recorded image density is deteriorated because it is necessary to add large amount of them to obtain a sufficient effect; and when used in a protective layer, the barrier ability reduces because these compounds are easily dissolved in plasticizer or oil; further the adherence of depositions or occurrence of sticking caused by the weak heat resistance, are pointed out.
As mentioned above, the method to provide a light resistance to a thermally sensitive recording medium by an ultraviolet absorbent has been carried into effect for long time, however, a sufficient method has not been developed yet.
The object of the present invention is to provide a thermally sensitive recording medium that has good appearance of unrecorded portion and color developing ability and further that the defects such as adherence of deposition or occurrence of sticking is prevented.
The inventors of the present invention have conduced an intensive study and developed the thermally sensitive recording medium of this invention. That is, the invention of claim 1 is a thermally sensitive recording medium that possesses a thermally sensitive color developing layer containing colorless or pale colored basic leuco dye and an organic color developer as main components on a substrate, wherein said thermally sensitive recording medium contains at least one kind of aqueous emulsion type polymer ultraviolet ray absorbent composed of an emulsion copolymerized compound of (a) ultraviolet ray absorbing monomer, (b) vinyl compound monomer that can copolymerize with said monomer, (c) hydrophilic monomer and (d) reactive emulsifier.
And, the invention of claim 2 is the thermally sensitive recording medium of claim 1, wherein the glass transition temperature Tg of said emulsion copolymerized compound is not less than 50xc2x0 C.
The invention of claim 3 is the thermally sensitive recording medium of claim 1 or claim 2, wherein said ultraviolet ray absorbing monomer which composes the aqueous emulsion type polymer ultraviolet ray absorbent is a compound represented by general formula I and/or general formula II, 
wherein, R1 represents hydrogen atom, alkyl group or alkoxy group of carbon number 1-6, R2 represents alkylene group or oxyalkylene group of carbon number 1-10, or not exists (in this case X combines with benzene ring directly), X represents ester bond, amide bond, ether bond or urethane bond and R3 represents hydrogen atom or lower alkyl group, 
wherein, R4 represents hydrogen, halogen or methyl group, R5 represents hydrogen or hydrocarbon group of carbon number 1-6, R6 represents alkylene group of carbon number 1-10 or oxyalkylene group or not exists (in this case Y combines with benzene ring directly), Y represents ester bond, amide bond, ether bond or urethane bond, R7 represents alkylene group of carbon number 1-8, aminoalkylene group, alkyl group possessing hydroxide group at side chain or not exists (in this case Y combines with C directly), and R8 represents hydrogen or lower alkyl group.
The invention of claim 4 is characterizing, said reactive emulsifier that composes the aqueous emulsion type polymer ultraviolet ray absorbent is a polymerizative vinyl compound.
As the (a) ultraviolet ray absorbing monomer, which composes the aqueous emulsion type polymer ultraviolet ray absorbent, to be contained in the thermally sensitive recording medium of this invention, any kind of monomer having ultraviolet ray absorbing ability can be used. However, desirably (a) is a polymerizative vinyl compound composed of a chemically bonded product of 2-hydroxybenzopehnone group in which an alkyl group or an alkoxy group of carbon number 1-6 can be substituted or 2-hydroxybenzotriazole group in which a hydrocarbon group of carbon number 1-6 can be substituted. As the concrete example of said polymerizative vinyl compound, compounds indicated by (1) or (2) can be mentioned, however not intended to be limited to these compounds, further these kinds of ultraviolet absorbing monomers can be used together with.
(1) 2-hydroxybenzophenone derivatives represented by general formula (I). 
wherein, R1 represents hydrogen atom, alkyl group or alkoxy group of carbon number 1-6, R2 represents alkylene group or oxyalkylene group of carbon number 1-10, or not exists (in this case, X combines with benzene ring directly), X represents ester bond, amide bond, ether bond or urethane bond and R3 represents hydrogen atom or lower alkyl group.
The monomer represented by general formula (I) can be obtained, for example, by reacting an ultraviolet ray absorbing compound having a functional group such as BPxe2x80x94R2xe2x80x94OH (BP: 2-hydroxybenzophenone structure) with a polymerizative vinyl compound having a functional group such as CH2xe2x95x90CR3xe2x80x94COOH, then introducing an ultraviolet ray absorbing compound residue into polymerizative vinyl compound by means of ester bond X (xe2x80x94COOxe2x80x94).
As the concrete examples of monomer compound represented by said general formula(I),
2-hydroxy-4-acryloyloxybenzophenone,
2-hydroxy-4-methacryloyloxybenzophenone,
2-hydroxy-4-(2-acroyloxy)ethoxybenzophenone,
2-hydroxy-4-(2-methacryloyloxy)ethoxybenzophenone,
2-hydroxy-4-(2-methyl-2-acroyloxy)ethoxybenzophenone or others can be mentioned
(2) 2-hydroxybenzotriazole derivatives represented by general formula (II), 
wherein R4 represents hydrogen, halogen or methyl group, R5 represents hydrogen or hydrocarbon group of carbon number 1-6, R6 represents alkylene group of carbon number 1-10 or oxyalkylene group or not exists (in this case, Y combines with benzene ring directly), Y represents ester bond, amide bond, ether bond or urethane bond, R7 represents alkylene group of carbon number 1-8, aminoalkylene group, alkyl group possessing hydroxide group at side chain or not exists (in this case, Y combines with C directly) and R8 represents hydrogen or lower alkyl group.
As the concrete examples of monomer compound represented by general formula (II),
2-[2xe2x80x2-hydroxy-5xe2x80x2-(methacryloyloxy)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-5xe2x80x2(acryloyloxy) phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-3xe2x80x2-t-butyl-5xe2x80x2-(methacryloyloxy)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-3xe2x80x2-methyl-5xe2x80x2-(acryloyloxy)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-5xe2x80x2-(methacryloyloxypropyl)phenyl]-5-chlorobenzotriazole,
2-[2xe2x80x2-hydroxy-5xe2x80x2-(methacryloyloxyethyl)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-5xe2x80x2-(acryloyloxyethyl)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-3xe2x80x2-t-butyl-5xe2x80x2-(methacryloyloxyethyl)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-3xe2x80x2-methyl-5xe2x80x2-(acryloyloxyethyl)phenyl]benzotriazole,
2-[2xe2x80x2-hydroxy-5xe2x80x2-(methacryloyloxypropyl)phenyl]-5-chlorobenzotriazole,
2-[2xe2x80x2-hydroxy-5xe2x80x2-(acryloyloxybutyl)phenyl]-5-methylbenzotriazole,
[2-hydroxy-3-t-butyl-5-(acryloyloxyethoxycarbonylethyl)phenyl]benzotriazole or others can be mentioned.
As the other vinyl compound monomer that can copolymerize with said ultraviolet ray absorbing monomer (b) (hereinafter shortened to copolymerization monomer), acrylonitrile, acrylic acid alkyl ester, metacrylic acid alkyl ester, alkyl vinyl ether, alkyl vinyl ester or styrene can be mentioned, and although the carbon number of these compounds are not restricted, desirable carbon number is 1-18. As the concrete examples of these compounds following compounds can be mentioned.
(1) acrylic acid alkyl ester, methacrylic acid alkyl ester.
methylacrylate, ethylacrylate, butylacrylate, 2-ethylhexylacryiate, octylacrylate, laurylacrylate, stearylacrylate, methylmethacrylate, butylmethacrylate, t-butylmethacrylate, 2-ethylhexylmethacrylate or others.
(2) alkyl vinylether.
methylvinylether, ethylvinylether, propylvinylether, stearylvinylether or others.
(3) alkylvinylester
vinylacetate, ethylvinyl, butylvinyl, 2-ethylhexylvinyl or others.
Copolymerization ratio of copolymerization monomer is 5-69% by weight to total weight of monomer.
In the present invention, by together use of a hydrophilic monomer (c), resistance to solvent and solubility with hydrophilic resin is further improved and a durable ultraviolet ray absorbing polymer film can be formed.
As a hydrophilic monomer, although any kind of monomer which has a hydrophilic reactive functional group can be used, desirably a hydroxy group containing monomer or a carboxylic acid containing monomer can be mentioned. As the concrete examples, following compounds can be mentioned.
(1) Hydroxy group containing monomer.
2-hydroxyethylmethacrylate, 2-hydroxyethylacrylate, hydroxypropyl methacrylate or others.
(2) Carboxylic acid containing monomer.
acrylic acid, methacrylic acid or others.
The desirable copolymerization ratio of a hydrophilic monomer is 1-20% by weight to total monomer weight. When the ratio exceeds 20 wt %, polymerization becomes difficult because of the problem of copolymerization with an ultraviolet ray absorbing monomer.
The aqueous emulsion type polymer ultraviolet ray absorbent used in this invention can be obtained by mixing each monomer with emulsifier in aqueous solvent and by emulsion polymerization using a aqueous polymerization initiator.
The emulsion copolymer is desirable to have a relatively high glass transition temperature (Tg), and when Tg is not less than 50xc2x0 C., good heat resistance of film (durability to sticking) can be obtained. Desirable Tg is not less than 70xc2x0 C. and more desirable Tg is not less than 80xc2x0 C. In the present invention, by adjusting the monomer ratio for emulsion copolymerization, the aqueous emulsion type polymer ultraviolet ray absorbent whose Tg is not less than 50xc2x0 C. can be easily obtained.
As the reactive emulsifier, any kind of conventional reactive emulsifier used in the usual emulsion polymerization that the radical polymerization is possible can be used, however, a polymerizative vinyl compound is desirably used. These reactive emulsifier can be used alone or can be used together with. Since these emulsifiers are introduced into the copolymer obtained at the emulsion polymerization, the bad influences to the water resistance and to the transparency of film, which usually occur in non-reactive emulsifier after the formation of film, are effectively prevented. As the concrete examples of the reactive emulsifier, Latemul S-180, S-180A, S-120A (products of Kao Co., Ltd.), Aqualon HS-10, HS-20, RN-20 series (Daiichi Kogyo Seiyaku Co., Ltd.), Eleminor JS-2 (Sanyo Kasei Industries Co., Ltd.) and Adekaria soap NE-30 (Asahi Denka Industries Co., Ltd.) can be mentioned. The Aqualon series comprises a RN and HS series, wherein RN are nonion type surface active detergents and HS are anion type surface active detergents having the general chemical structure R-phenyl(CHxe2x95x90CHCH3)O(CH2CH2O)mSO3NH4. The Latemul series compounds contain the general chemical structure MO3SCHxe2x80x94(H2Cxe2x80x94COOR)xe2x80x94COOCH2CO(OH)CH2OCH2CHxe2x95x90CH2, wherein R is C12 and C18F1, and M is Na or NH4. From the view point of color developing ability of the thermally sensitive recording medium, anion type reactive emulsifier is desirably used. The desirable amount of the reactive emulsifier to be used is 0.5-5 weight parts to 100 weight parts of total monomer amount. If the amount of use is over than 5 weight parts, the effect to a basic leuco dye is too strong and is a ground to interfere the color developing, and the water resistance of the film is weakened.
Generally, the conventional emulsifier, which is used for the emulsification, surrounds and contacts the whole surface of emulsified material. Meanwhile, the reactive emulsifier used in this invention reacts with the emulsified material and is introduced into it, and not remains the original form of emulsifier. Therefore, the aqueous emulsion type polymer ultraviolet absorbent of this invention becomes soap free, and has a strong point that does not influence badly on the color developing material.
As the polymerization initiator, peroxide such as persulfate, percarbonate or perborate, or azo type aqueous polymerization initiator such as
2,2xe2x80x2-azobis[2-(2-imidazoline-2-yl)propane]hydrochloride,
2,2xe2x80x2-azobis(2-methylpropionamidine)hydrochloride,
2,2xe2x80x2-azobis[N-(2- hydroxyethyl)-2-methylpropionamidine]hydrochloride,
2,2xe2x80x2-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidine-2-yl)propane]hydro-chloride and others can be used. Desirable amount of aqueous polymerization initiator is 0.05-1 wt % to total weight of monomer.